It is known to measure the jitter modulation of a digital signal by a phase comparison of first pulses derived from certain pulse flanks of a jitter-associated digital signal and second pulses of a jitter-free reference clock which are derived from the bit repetition frequency of the jitter-associated digital signal.
One pulse starts a linearly rising ramp generator while the other pulse determines sampling points of the ramp signal and the sampled values of the ramp signal are held. An alternating component of the sampled and held ramp signal is proportional to the jitter-modulation and a direct component of the sampled and held ramp signal establishes a setting value for the generation of the jitter-free reference frequency.
Methods and circuits utilizing the above-described principles are utilized in phase modulation and demodulation with respect to phase meters which are also used in the jitter-measurement and technology for digital transmission purposes (see CCITT 0.171 and definitions).
Jitter measurements are effected generally on binary signals, i.e. signals with a substantially rectangular wave form with respect to time.
For the recovery (demodulation) of the phase-time function .phi. (t) at the instantaneous positions of the flanks, basically two types of phase comparators are used.
Phase comparators of a first type convert the digital signal containing the phase-time function .phi. (t) with the aid of digital circuit units with the assistance of a jitter-free reference clock signal of identical bit frequency to a pulse duration modulated signal from which the desired phase-time function .phi. (t) is recovered by low-pass filtering.
Such phase meters are described by Tietze Schenk, HALBLEITER-SCHALTUNGSTECHNIK Volume 7, 1985, Springer Verlag, Section 26.4.3., pages 824 to 827. Phase comparators of a second type convert the phase difference between the phase-time function .phi. (t) containing digital signal and a jitter-free reference clock signal of identical bit frequency with the aid of a sampling phase comparator into a pulse-amplitude modulated (PAM) signal which gives the phase-time function directly. In this case, a subsequent low pass filtering is not mandatory.
Phase comparators of this second type operate generally by utilizing the ramp of a sawtooth signal of the reference frequency which is sampled by a pulse derived from the digital signal to be measured. The sequence of the resulting sampling values or scanning values provides the phase-time function .phi. (t).
An example of a phase comparator which functions generally in accordance with the second type of comparator, although using a sinusoidal reference signal, is found in Tietze Schenk, OP.CIT. Section 26.4.3., pages 819 to 822.
Phase comparators of the second type have indeed a good linearity, although they have a narrow dynamic range. The latter is generally significantly below .+-..pi.. With higher bit frequencies, the dynamic range can be below .+-.(.pi./2). If larger measuring ranges are desired, greater ramp lengths are necessary which, however, give rise to an attenuation of the flank density of the digital signal. A process of this type can be carried out only with difficulty for zero-related digital signals.
Phase comparators of this second type do have, however, the significant advantage that they are suitable for directly operating on zero-related digital signals, since the detected phase (or an equivalent voltage value) can be stored until the arrival of the next bit flank even when these bit flanks are not equidistant.
They also have the further important advantage that they can measure the peak values of the generated PAM signal directly so that a theoretically maximum measuring band width can be realized.
A further important advantage of the phase comparator of the second type is that in the peak-value measurement of the PAM signal (without low-pass filtering), no pattern-dependent additional measurement error can arise.